"Mice that are homozygous for the targeted mutant Smn1 allele and carry the SMN2 transgene exhibit symptoms and neuropathology similar to patients afflicted with type I proximal spinal muscular atrophy (SMA). In the initial characterization by the donating investigator, mice were either stillborn or survived 4-6 days. Mice that died at or shortly after birth were slightly smaller (1.33 g. vs. 1.51 g.) than normal littermates. Mice that survive for several days are indistinguishable from normal littermates in the first 48 hours, after which they exhibit decreased suckling and movement, labored breathing and tremoring limbs. Mice succumbing at this later time point are noticeably smaller than normal littermates (1.47 g vs. 4.59). A bell-shaped trunk is also noticeable in affected mice, presumably from intercostal muscle weakness, a characteristic of type I SMA. Histological analysis indicates that affected mice that survive to day 5 exhibit a loss of motor neurons from spinal cord (35%) and facial nucleus (40%). A large number of cells with pyknotic nuclei are observed in these tissues. Immunohistochemical analysis indicates low-level expression of the SMN2 protein in the tissues examined (brain, liver, spinal cord) and an absence or near absence of intranuclear aggregates of the SMN protein ('gems'). The donating investigator reports that muscle fibers (quadriceps and gastrocnemius assayed) are atrophied, a characteristic observed in SMA patients. Homozygous mice bearing the Smn1 targeted mutation without a copy of the SMN2 transgene display an embryonic lethal phenotype with developmental arrest occurring prior to implantation. "